When connecting together pipes 1, 2 that carry steam, it is customary to connect the pipes 1, 2 using bolted flanges 11, 12, as shown in FIG. 1. The ends of the pipes 1, 2 form matching flanges 11, 12 with bolt holes provided around each flange, through which bolts 13, 14 are provided, connecting the pipes 1, 2. The steam passing through the pipes 1, 2 is under pressure, and acts to force the joint apart. The tightened bolts 13, 14 resist this steam pressure, sealing the joint at the abutting raised faces of the flanges 11, 12 and allowing the steam to pass across the joint.
The bolts 13, 14 are usually made from alloy steel, but due to the use of alloy steel they have a limited life when exposed to conventional superheated steam turbine temperatures (up to 565° C.). Exposed to these conditions, the bolts suffer creep relaxation and eventually the seal formed at the joint by the bolted flange weakens so that steam can escape at the joint, as it is no longer fully sealed.
At higher temperatures, i.e. above 565° C., heat resistant alloys such as Nickel-Cobalt nimonic alloys have to be considered for use in making the bolts. However, the disadvantage of making the bolts using this material is that they are expensive.
One solution that has been proposed is the use of threaded connectors 22 to secure an annular cover 20 over the abutting ends of a curved pipe 1 and a straight pipe 2 as shown in FIGS. 2a and 2b. FIG. 2a shows the assembly prior to the fitting of the cover 20. FIG. 2b shows the assembly after fitting the cover 20.
Prior to fitting the cover 20, as shown in FIG. 2a, each axial end of the cover 20 is held in place over the straight pipe 2 using two retaining rings 21, 28. There are a plurality of axially oriented threaded bores in the radially outer portions of the two retaining rings 21, 28, through which pass respective adjustable threaded connectors 22. The adjustable threaded connectors 22 are threaded cylindrical bars, and secure the two retaining rings 21, 28 in place. The adjustable threaded connectors 22 are mounted on the segmented projecting rings 23, 24, through a plurality of axially oriented threaded bores in the segmented projecting rings 23, 24, and are fixed at either side of the bores with bolts 27. The radially inner ends of the segmented projecting rings 23, 24 are mounted in recessed grooves 25, 26 provided in the respective pipes. The recessed grooves 25, 26 are located a short distance away from the respective ends of the pipes 1, 2, towards the main body of the pipes 1, 2. The projecting segmented rings 23, 24 in the recessed grooves 25, 26, connected by the threaded connectors 22, fix the assembly to both pipes 1, 2 and assist in holding the respective ends 20c, 20g of the pipes 1, 2 together.
The pipes 1, 2 have flanges 29a, 29b near their respective contacting ends, and these flanges 29a, 29b extend axially along each pipe but not to the end of each pipe. When the pipes 1, 2 are abutting, a groove 29 is defined by the axial gap created between the pipe flanges 29a, 29b when the ends of the pipes 1, 2 abut. A key or keys 20e is inserted into the groove 29 and is fixed using bolts. The function of the key 20e is to fix the orientation of the pipes 1, 2 relative to one another by transmitting any torque from one pipe to another, preventing relative rotational movement.
The cover 20 has, on the part of its radially inner facing surface that will lie over curved pipe 1, a female threaded connector surface 20a. Curved pipe 1 has a co-operating male threaded connector surface 20b located on the radially outer facing surface of the end portion of the pipe 1 to be covered by the cover 20.
To seal the pipes 1, 2 together, the cover 20 is moved from its position on the straight pipe 2 to cover both pipes 1, 2. This is done by rotating the threaded connectors 22, causing the retaining rings 21, 28 to move axially, until the female threaded connector surface 20a on the inside surface of the cover 20 begins to engage the male threaded connector surface 20b on the outside surface of the ends of curved pipe 1. Once these threaded surfaces 20a, 20b are engaged, they are tightened by rotating the cover 20 about the axial centre line of the pipes 1, 2. This is done using a generic tool suitable for rotating the cover co-operating with a bolt 20d (FIG. 2b) projecting from the cover 20. Thread engagement and tightening is aided by the cover 20 being held concentric to the centreline of the pipes 1, 2 by retaining rings 21 and 28. For this tightening to take place, cover 20 has to be rotated about the axial centre line of the pipes 1, 2 whilst retaining rings 21,28 slide axially on threaded connectors 22. The cover 20 is formed having a radially inward facing flange, provided on the inner surface of the end of the cover 20 that lies over straight pipe 2 when the cover 20 is in place connecting the pipes 1, 2. After fitting the cover 20, as described above and shown in FIG. 2b, the raised face of the inward facing flange on the cover 20 abuts the raised face of flange 20f on the straight pipe 2, preventing further axial movement.
Sealing occurs at the raised faces 20c, 20g (FIG. 2a) of the flanges of the pipes 1, 2 due to the compressive force from cover 20.
However, this arrangement has the problem that it is complex to install and not easily removable once installed, requiring a pipe to be cut if it is to be removed. This is because the large diameter threads 20a, 20b are difficult to undo. This is especially the case after the joint has been in operation for any considerable period of time as scale will build up in the assembly. This means that large amounts of torque need to be applied in order to undo the large bolts used, due to the increased friction caused by the scale. Also, for the assembly to work it is required that the pipes are adjacent and of the same diameter.